Editing River delta (section)

== Formation ==
[[File:Delta Formation.svg|thumb|A delta forms where a river meets a lake.<ref>{{Cite web |url=https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA19071 |title=How a Delta Forms Where River Meets Lake |website=[[Jet Propulsion Laboratory]] |date=2014-08-12 |access-date=2017-12-12 |archive-date=2017-12-12 |archive-url=https://web.archive.org/web/20171212160758/https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA19071 |url-status=live }}</ref>]] 
River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a  [[reservoir]]. When the flow enters the standing water, it is no longer confined to its [[stream channel|channel]] and expands in width. This flow expansion results in a decrease in the [[flow velocity]], which diminishes the ability of the flow to [[sediment transport|transport sediment]]. As a result, sediment drops out of the flow and [[deposition (geology)|is deposited]] as [[alluvium]], which builds up to form the river delta.<ref>{{Cite web|url=http://pasternack.ucdavis.edu/research/projects/tidal-freshwater-deltas/tfd-modeling/|title=Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Modeling|website=pasternack.ucdavis.edu|language=en|access-date=2017-06-12|archive-date=2018-09-30|archive-url=https://web.archive.org/web/20180930093758/http://pasternack.ucdavis.edu/research/projects/tidal-freshwater-deltas/tfd-modeling/|url-status=live}}</ref><ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |pages=289–306 |edition=4th}}</ref> Over time, this single channel builds a deltaic lobe (such as the bird's-foot of the Mississippi or [[Ural (river)|Ural river]] deltas), pushing its mouth into the standing water. As the deltaic lobe advances, the [[gradient]] of the river channel becomes lower because the river channel is longer but has the same change in elevation (see [[slope]]).

[[File:Sacramento Delta at flood stage, 2009.jpg|thumb|[[Sacramento–San Joaquin River Delta|Sacramento–San Joaquin (California) Delta]] at flood stage, early March 2009]]

As the gradient of the river channel decreases, the amount of [[shear stress]] on the bed decreases, which results in the deposition of sediment within the channel and a rise in the channel bed relative to the [[floodplain]]. This destabilizes the river channel. If the river breaches its natural [[Levee|levees]] (such as during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a steeper, more stable gradient.<ref name="SlingerlandSmith">Slingerland, R. and N. D. Smith (1998), "Necessary conditions for a meandering-river avulsion", ''Geology'' (Boulder), 26, 435–438.</ref> Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up a mature delta with a [[distributary]] network.

Another way these distributary networks form is from the deposition of [[mouth bars]] (mid-channel sand and/or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth bar, which splits the river into two distributary channels.{{sfn|Boggs|2006|p=295}}<ref>{{cite book |last1=Leeder |first1=M. R. |title=Sedimentology and sedimentary basins: from turbulence to tectonics |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex, UK |isbn=9781405177832 |page=388 |edition=2nd}}</ref> A good example of the result of this process is the [[Wax Lake Delta]].

In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan. The more often the flow changes course, the shape develops closer to an ideal fan because more rapid changes in channel position result in a more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's feet, are examples of rivers that do not [[Avulsion (river)|avulse]] often enough to form a symmetrical fan shape. [[Alluvial fan]] deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape.

Most large river deltas discharge to intra-cratonic basins on the trailing edges of passive margins due to the majority of large rivers such as the [[Mississippi River|Mississippi]], [[Nile]], [[Amazon River|Amazon]], [[Ganges]], [[Indus]], [[Yangtze]], and [[Yellow River]] discharging along passive continental margins.<ref name=Milliman>{{cite journal |last1=Milliman |first1=J. D. |last2=Syvitski |first2=J. P. M. |year=1992 |title=Geomorphic/Tectonic Control of Sediment Discharge to the Ocean: The Importance of Small Mountainous Rivers |journal=[[The Journal of Geology]] |volume=100 |issue=5 |pages=525–544 |doi=10.1086/629606 |jstor=30068527 |bibcode=1992JG....100..525M |s2cid=22727856 }}</ref> This phenomenon is due mainly to three factors: [[topography]],  basin area, and basin elevation.<ref name=Milliman/> Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate over time to form large river deltas. Topography along active margins tends to be steeper and less widespread, which results in sediments not having the ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow [[continental shelf]].

There are many other lesser factors that could explain why the majority of river deltas form along [[Passive margin|passive margins]] rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, and volcanic activity resulting in delta formation to exist closer to the sediment source.<ref name=Milliman/><ref name=Goodbred>{{cite journal |last1=Goodbred |first1=S. L. |last2=Kuehl |first2=S. A. |year=2000 |title=The significance of large sediment supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of the Ganges-Brahmaputra delta |journal=Sedimentary Geology |volume=133 |issue=3–4 |pages=227–248 |doi=10.1016/S0037-0738(00)00041-5|bibcode=2000SedG..133..227G }}</ref> When sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel [[Avulsion (river)|avulsion]], delta lobe switching, and auto cyclicity.<ref name=Goodbred/> Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment.<ref name=Milliman/> However, the sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches.<ref name=Milliman/>

At the mouth of a river, the change in flow conditions can cause the river to drop any sediment it is carrying. This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels. Landforms at the river mouth drastically alter the geomorphology and ecosystem.<ref>{{Cite web |date=2019-06-22 |title=13.4: Landforms of Coastal Deposition |url=https://geo.libretexts.org/Bookshelves/Oceanography/Introduction_to_Oceanography_(Webb)/13:_Coastal_Oceanography/13.04:_Landforms_of_Coastal_Deposition |access-date=2025-02-15 |website=Geosciences LibreTexts |language=en}}</ref>